1. Field of the Invention
This invention relates to an air-fuel ratio-control system for internal combustion engines, which controls the air-fuel ratio of a mixture supplied to the engine in a feedback manner responsive to an output from an air-fuel ratio sensor arranged in the exhaust system of the engine.
2. Prior Art
There is conventionally known a method of controlling the temperature of an air-fuel ratio sensor used in the air-fuel ratio feedback control of an internal combustion engine, for example, from Japanese Patent Publication (Kokoku) No. 7-78484, in which the air-fuel ratio sensor is provided with an electric heater for controlling the temperature thereof, and the sensor temperature is detected based on the internal resistance of the sensor, to thereby control an amount of electric current flowing in the heater according to the detected sensor temperature. Further, there is also known a method of determining activation of the air-fuel ratio sensor by measuring the internal resistance of the sensor as a parameter representative of the sensor temperature, and determining whether or not the sensor temperature has reached a predetermined value, i.e. whether or not the air-fuel ratio sensor has been activated, based on the measured resistance value.
To satisfy stricter regulations of exhaust gas in recent days, there is an increasing necessity to start the air-fuel ratio feedback control as soon as possible after the start of the engine. Therefore, prompter activation of the air-fuel ratio sensor is desired.
One way to more promptly activate the air-fuel ratio sensor is to heat the air-fuel ratio sensor by a heater as employed by the above known method and further improve the performance of the heater. This method, however, can not only incur increased power consumption and increased costs, but also provide a higher possibility of overshooting of the temperature (overheating) of the sensor during heating by the heater.
On the other hand, if the predetermined value for determining whether or not the air-fuel ratio sensor has been activated is set to a lower value to start the air-fuel ratio feedback control earlier after the start of the engine, there arise inconveniences that immediately after the sensor is determined to be activated, the sensor output characteristic can vary so that the sensor output deviates from the actual air-fuel ratio, and the sensor can show a poor output response to a change in the air-fuel ratio. As a result, desired air-fuel ratio feedback control performance cannot be achieved.